Low thermal expansion titania doped fused silica glasses (hereinafter silica-titania glasses) frequently can have a brownish appearance (due to the presence of Ti+3) and are not known for high transmission. However, there exists a need for low thermal expansion silica-titania glass with improved transmission for uses such as Imprint Microlithography.
While low expansion silica-titania glasses such as ULE® Glass (Corning Incorporated) have a low thermal expansion behavior that can be tailored, the thermal expansion behavior changes with temperatures to values that are undesirable for certain products and applications. For example, it is desirable that EUVL (extreme ultraviolet lithography) reflective optics and masks experience minimum distortion during their use, the use temperatures typically being in the range from 20° C. to 80° C. or higher. If the expansivity curve for the glass used to EUVL masks and optics could be made flatter there would be less in-use distortion in the masks and lens elements. EUVL system designers need to carefully budget thermal distortions in order for the lithography system to remain at the specified performance level as it heats up during a production run. The present disclosure identifies silica-titania glass that has improved expansivity (a flatter expansivity curve) and also has improved transmission characteristics. This glass enables the system designers to make EUVL systems with higher throughput, better overlay and resolution.
At the present time the process of making large boules of silica-titania glass can result in a glass where the CTE variation across the boule ranges from 3 to 10 ppb/° K. In addition, the absolute CTE of such glasses can only be controlled to +/−3-5 ppb/° K. As a result, glass removed from various parts of a boule can have some CTE variation. The present disclosure discloses a means of adjusting the absolute CTE via a change in the rate of anneal which simultaneously leads to improved transmission properties as well as an adjustment to the absolute CTE.